Production of dislocation-free silicon single crystals



Sept. 27, 1966 P. H. HUNT PRODUCTION OF DISLOCATION-FREE SILICON SINGLE CRYSTALS Filed Oct- 15, 1963 Fllg'j United States Patent 3,275,417 PRODUCTION 0F DISLOCATION-FREE SILICON SINGLE CRYSTALS Patrick H. Hunt, Richardson, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Get. 15, 1963, Ser. No. 316,347 4 Claims. (Cl. 23-273) This invention relates to a float zone process for forming silicon single crystals and more particularly to the control of a float zone crystal growth in which control is maintained over a molten zone and a freezing solid-liquid interface to minimize crystal dislocations.

In the manufacture of silicon semiconductor devices, high quality single crystals of silicon are necessary as starting materials. In accordance with one known method of growing crystals, a silicon seed is dipped into a molten pool of silicon contained in a quartz crucible and then slowly withdrawn. It has been found that resulting crystals contain approximately atoms per cc. of oxygen which is derived from reaction of the molten silicon with the quartz crucible. This leads to undesirable heat treating effects as to the resistivity of the silicon body during subsequent processing. Thus, it is desirable to eliminate the high oxygen content.

In float zone processes of the type generally disclosed in Keller Patent No. 2,992,311, a molten zone is caused to move along the length of an elongated silicon rod which is held in a vertical position. This method has the ad vantage of producing low oxygen content crystals, but has been found to be disadvantageous in that any crystal of reasonable diameter, snchas above about one centimeter, contains a large number of dislocations. The dislocations lower the minority carrier lifetime and further act as preferential diffusion paths for impurities during further processing and for the foregoing reasons are highly undesirable. In Dash Patent No. 2,961,305, contact between a molten zone and a single silicon crystal is established at the top pedestal silicon which is longitudinally slotted. This slotting process and the fact that only small diameter crystals can be grown from the pedestal makes the system somewhat limited as a production operation although structurally perfect crystals may be obtained.

In accordance with one aspect of the present invention, there is provided a system for transfer to the form of a dislocation-free crystal of semiconductor material from a rod supported at the bottom thereof in a controlled atmosphere within a non-conductive tube onto a seed crystal supported for rotation above and at the axis of the rod. Means including a single-turn, inductive heating member encircles the rod outside the tube and is mounted for relative movement with respect to the rod for production of a molten zone in the rod when energized by high frequency alternating current. A cooled, singleturn, shorted focusing coil encircles the axis of the rod immediately below the heating member to control the molten zone for limiting the character and the height thereof.

In a more specific aspect, the focusing coil is in the form of an upwardly pointing truncated cone having a central aperture through which the rod passes. This coil is positioned within the normal field produced by excitation of the R.F. coil. The focusing coil is a continuous cone of highly conductive material and is thus shorted, by which is meant closed. That is, it limits or modifies the shape of the field produced by the R.F. coil by absorbing power from part of the field. The shape of the focusing coil and its position relative to the heating coil determine the shape and extent of the zone heated 3,275,417 Patented Sept. 27, I966 ice by induction. Thus the coil is said to focus the R.F. field.

In accordance with a further aspect of the invention, there is provided a method for crucible-free float zone forming of a dislocation-free crystal from a rod of semiconductor material. The rod is supported from the bottom in substantially vertical orientation in an inductively coupled R.F. field to form a molten zone in the rod which is sharply limited on the lower boundary thereof. A single crystal silicon seed is dipped into the molten zone and then, while under rotation, the seed is withdrawn from the zone at a rate initially substantially in excess of the movement of the rod upward through the field and then at a rate substantially corresponding with the rate of movement of the rod through the field.

For a more complete understandig of the present invention and for further objects and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a diagrammatic view of an apparatus embodying the present invention;

FIGURE 2 is a perspective view of the coil 13 and its support; and

FIGURE 3 is an enlarged View of a seed and molten zoneshortly after initiation of crystal growth.

FIGURE 1 illustrates a system for float zone crystal growth in accordance with the present invention. A rotating single crystal seed 10 of silicon initially is dipped into a molten pool formed on the top of a rod 11 of silicon. Radio frequency power coupled to the silicon rod by a single-turn, closely-coupled coil 13 is adjusted until the tip of the seed melts, forming a continuous molten zone between the rod 11 and the seed 10. The seed 10 is then withdrawn from the melt, and the R.F. power and the withdrawal speed are adjusted so that a long neck 14 is formed below the seed 10. The neck 14 is of diameter smaller than that of the seed. After the neck 14 is of length of at least about one to two centimeters, the R.F. power and the seed withdrawal rate are then changed gradually to preset values, during which time a transition zone 15 is formed. Predetermined values of withdrawal and feed are reached which determine the diameter of the newly-formed crystal 17 above the melt zone 16.

In accordance with the present invention, control is maintained over the heating zone and particularly over the geometry of the R.F. field sharply to limit the lower boundary of the field. This permits control over the molten zone thereby to assure a minimum occurrence of dislocations.

The system for carrying out crystal growth in accordance with the preferred embodiment of the invention is shown in FIGURE 1. In this embodiment, the lower end of rod 11 is supported in a chuck, which chuck is supported by a bottom mounting ring 20. Ring 20 is supported at the lower end of a rectangular frame 21 having two upper crossbars 22 and 23. The frame 21 is supported at opposite sides thereof on lead screws such as the screw 25 and by followers 26 and 27. The lead screw 25 is supported at its lower end by a member 30 of a fixed frame or support. A second lead screw (not shown) is similarly supported at its lower end. A motor 31 is illustrated as connected to lead screw 25 by way of bevel gears 32. A similar driving arrangement is provided for the other lead screw. Preferably, the motor 31 will drive both lead screws so that the frame 21 may be precisely controlled in its movement upward relative to coil 13.

The seed 10, of rectangular cross section, is mounted in a chuck 35 and is solely supported in chuck 35 as to be movable relative to the frame 21. The chuck 35 is supported at the lower end of a shaft 36. The shaft 36 is journaled in an upper housing member 37 and is provided with a drive pulley 38. The shaft 36 may be rotated by way of belt 39 driven from a motor 40. The motor 40 is mounted on a cross plate 41 which extends between a pair of followers 42 and 43. Followers 42 and 43 are mounted on lead screws 44 and 45, respectively. The lower ends of the lead screws 44 and 45 are journaled in the crossbars 22 and 23. A pulley 46 is mounted on the lead screw member 44 between crossbars 22 and 23 and is coupled by way of belt 47 to a first pulley on the shaft of a motor 48. In a similar manner, a second pulley 49 mounted on the lower end of the lead screw 45 is coupled by way of a belt 50 to a second pulley on the shaft of the motor 48. Pulleys 46 and 49 are keyed t the lower ends of the lead screws 44 and 45, respectively, for transmission of driving forces thereto. As the motor 40 is energized, the shaft 36 carrying the chuck 35 is caused to rotate in order to rotate the seed 10. As the motor 48 is energized, the lead screws 44 and 45 are rotated, causing the followers 42 and 43 tobe raised or lowered, depending upon the direction of rotation of the motor 48. Thus, the seed may be raised or lowered relative to the upper end of the rod 11.

The housing 37 in which the shaft 36 is journaled is coupled by way of a flexible bellows 51 to the upper end of a mounting ring 52. The shaft 36 may be slidably supported in a bearing in the mounting ring 52. An auxiliary hand-powered drive arrangement is provided for raising and lowering the seed 10. A crank wheel 53 is coupled by bevel gears 54 to the shaft of motor 48 so that an adjustment can be made as to the elevation of the seed 10 without energizing motor 48.

A quartz tube 60 is clamped in the rings 20 and 52 and is coaxial with the rod 11. Thus, all of the structure carried by frame 21 is movable relative to coil 13 so that a molten Zone initiated in rod 11 may be caused to travel along the length of the rod 11 by raising the frame 21. Preferably, the travel of the frame 21 is carefully controlled as to be of constant speed so that mechanically induced aberrations may be avoided.

In accordance with the present invention, the character of the freezing interface between the melt zone 16 and the newly-formed crystal 17 and the starting rod 11 is maintained horizontal and fiat by control of the molten zone. This is accomplished, in accordance with the present invention, by control of the heating pattern from the coil 13 and by the movement of the newly-formed crystal 17 relative to the crystal 11. More particularly, the coil 13 is a hollow, single-turn copper loop such as shown in FIGURE 2. It is mounted on a pair of brackets 70 and 71 which are insulated one from the other. Coil 13 is flow-connected at the ends of the single turn to a source of cooling water, the flow of which is maintained through the coil. Coil 13 is also connected to a source of R.F. power at frequency of about 4 megacycles and capacity of about 25 kilowatts. In addition to this, focusing coil 72 is mounted immediately below the coil 13 and is in the form of a hollow, truncated cone having a center bore through which the tube 60 extends. The focusing coil 72 is made of a highly conductive material such as copper, and is provided with a pair of flow channels 73 and 74 leading thereto. Flow channels 73 and 74 provide for flow of cooling water through the focusing coil 72. The coil is placed immediately adjacent to and immediately below the coil 13 and serves to rob power from the field produced by excitation of the coil 13. This serves to control the length of the molten zone and limits its shape. The control over the configuration of the freezing zone above the coil 13 has been found to be controllable by this means so that the crystal 17 may be grown above rod 11 of diameter equal the diameter of rod 11 without the molten zone 16 falling out and with substantial freedom of dislocations.

A continuous flow of an inert gas, such as argon, is

maintained through the tube 60 during the crystal growing cycle. Argon is introduced into the lower end of the tube 60 by way of a channel 75. Argon flows from the upper end of the system by way of channel 76.

The growing cycle is initiated by playing a flame from a hydrogen torch '77 onto the wall of the tube 60 in a region preferably somewhat below the upper end of the rod 11. As the rod 11 is heated, the coil 13 is energized from a radio-frequency generator so that coil 13 becomes electromagnetically coupled to the rod 11. The coupling is such that the rod may be made molten by increasing the energy level of the field from coil 13. The flame from torch 77 is then turned olf. The rod 11 is then lowered so that the R.F. coupling zone is at the top of rod 11. The R.F. power is then increased.

As the rod 11 becomes molten, the seed 10 is lowered so that the lower end thereof is immersed in the molten pool on the upper end of the rod 11. Surface tension of the molten zone 16 and the electromagnetic field from the coil 13 maintains the integrity of the molten zone preventing its falling out. The seed 10 is then rotated at a relatively slow speed and is slowly withdrawn from the upper surface of the molten pool on rod 11. The motors 31 and 48 are adjusted as to speed so that the crystal initially grown onto the seed 10 is necked down to a smaller diameter than the seed 10 and is permitted to maintain the relatively small diameter until it attains a length of several centimeters. At this time, the rate of movemen of the seed 10 relative to the rod 11 is reduced gradually so that the transition zone 15 is formed. The transition zone 15 freezes as it is drawn from the molten zone 16. In operation, it has been found that the movement of the seed 10 relative to the rod 11 may be discontinued so that the newly-formed section 17 will have the same diameter as the original rod 11. The motor 31 then continues to drive the frame 21 upward, carrying with it the rod 11 as encased within the quartz tube 60. Thus, the molten zone 16 moves downward through the entire length of the rod 11 with the newly-formed crystal 17 freezing progressively above the molten zone as the body moves away from the zone controlled by the coil 13. A single pass beginning at the top of the rod 11 and terminating at the bottom of the crystal has been found to produce a crystal 17 which is free from dislocations and has a relatively low oxygen content and high minority carrier lifetime.

The traverse of the molten zone 16 down the length of the rod 11 may be of the order of about one hundred and fifty millimeters per hour with no differential pull rate and with the rotation of the seed 10 and the newly-formed crystal 17 in the range of from about two (2) to twenty (20) rpm. The rotational speed preferably is in the lower end of the latter range. Crystals may be grown having diameters of two (2) to three (3) centimeters or more while maintaining the freedom from dislocations. The flow rate of gas by way of channels 75, 60 and 76, at a rate of about one-half liter per minute, has been found to be satisfactory. Higher flow rates are employed when the diameter of the newly-formed crystal section 17 reaches a diameter of about 3.3 millimeters. In the latter range, flow rates of one to 1.5 liters per minute will be employed.

In a typical example, the rod 11 may have a diameter of about twenty-eight (28) millimeters. The quartz tube 60 would have an inner diameter of thirty-two (32) millimeters and an outer diameter of thirty-four (34) millimeters. The coil 13 is about five (5) millimeters tube size and of diameter such that the quartz tube 60 is free of the coil 13 but with minimum spacing therebetween. The focusing coil 72 is of the same inner diameter as the inner diameter of the coil 13. The upper surface of the coil 72 was spaced about ten (10) millimeters below the lower surface of the coil 13. Coil 72 was of the order of one hundred and twenty-seven (127) millimeters in diameter at the bottom and slightly larger than the coil 13 at the top thereof.

In the foregoing example, the rod 11 was about twentyeight (28) millimeters in diameter. The travel of the rod through the coil 13 was maintained at about one hundred and fifty (150) millimeters per hour. In instances where larger diameter crystals have been employed, such as thirty-three (33) millimeter crystals, the speed has been lowered somewhat to about one hundred and fifteen (I15) millimeters per hour in order to assure a zone which is completely molten, and to avoid freezing up during the pulling operation. The traverse rate will thus depend upon the size of the rod employed and upon the power in the RF. field. It should therefore be understood that the foregoing is given by way of example and not by way of limitation.

In order to form dislocation-free crystals, it is necessary that the seed be thoroughly wetted in the molten zone on top of the rod 11 before beginning to pull the crystal. A guide by which the operation may be evaluated is illustrated in FIGURE 3. The seed 10 is withdrawn from the molten top of rod 11 to form the neck 14 of reduced diameter to assure crystal structure of a large diameter which is dislocation-free. Crystal growth preferably is initiated when the lower end-of seed 10 is of the shape illustrated in FIGURE 3. More particularly, as the seed 10 rotates and becomes wetted in the melt, the corners of the seed tend to form cusps with downwardly pointed corners. When the seed 10 is shaped generally in the form indicated in FIGURE 3, the pulling operation may then be commenced.

While not shown in the drawings, the system includes electrical controls for the speeds of motors 31, 40, and 48 as well as for control of the level of power from a suitable R.F. generator for the coil 13 of FIGURES 1 and 2. Such controls are well-known in the art.

It is to be understood that the system operates to transfer semiconductor material from the bottom supported rod 11 into the form of a dislocation-free crystal. The operation is carried out in the argon atmosphere in the tube 60. The transfer, by way of a molten zone of controlled configuration, is onto a single crystal seed rotatably supported above the molten zone. The singleturn annular heating coil 13 encircles the rod 11 outside the tube 60 and is energized by high frequency alternating current to create the molten zone in the rod 11. The single-turn, shorted focusing coil 72 encircles rod 11 immediately below the heating coil 13 to rob power from the RF. field at the lower boundary to limit the height of the molten zone and thus control the molten zone.

The foregoing description has related primarily to the growth of crystal structures of silicon base materials. It will be recognized that other semiconductor materials may be similarly treated to produce dislocation-free structures. In the growth of germanium crystals, the primary problem attendant to the production of dislocation-free silicon crystals is not present. Thus, germanium can be grown from melts contained in quartz crucibles. Nevertheless, the present invention may be employed for production of germanium bodies which are dislocationfree, if desired. Since the density of germanium is somewhat greater than that of silicon, the diameter of the float zone would have to be maintained somewhat smaller in order to maintain its integrity. In providing an inert atmosphere in which the molten zone is maintained, argon has been used and has been specified in the foregoing description. However, any inert gas which will not react with silicon at the temperatures involved would be suitable. Helium, neon, and the like, as well as hydrogen, are satisfactory.

Having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art and it is intended to cover such 6 modifications as fall within the scope of the appended claims.

What is claimed is:

1. A system for crucible-free float zone forming of a rod of semiconductor material which comprises:

(a) means including a single-turn R.F. induction coil positioned in a horizontal plane,

(b) a hollow quartz tube extending through said coil,

(0) a conductive ring encircling said tube immediately below said coil and adapted for flow of liquid coolant therethrough as a body which is a truncated cone, the conical surface of which radiates outwardly from said R.F. induction coil,

(d) first supporting means for said rod at the bottom of said tube,

(e) second supporting means for a single crystal seed of semiconductor material in the upper end of said tube,

(f) means for rotating said second supporting means,

(g) means for moving said second supporting means relative to said first supporting means along the axis of said tube,

(b) means for moving said first supporting means relative to said coil, and

(i) means for flowing a predetermined gas through said tube.

2. A system for forming a dislocation-free crystal from a bottom-supported cylindrical semiconductor rod in vertical orientation in a controlled atmosphere which comprises:

(a) means including a single-turn, annular, heating member encircling said rod and mounted for relative movement between said heating member and said rod, said heating member being of hollow, tubular configuration energized by high frequency alternating current,

(b) auxiliary means for initially heating said rod to permit the coupling of power from said heating membe;I to said rod to establish a molten zone in said ro (c) a truncated cone, the conical surface of which radiates outwardly from said heating member, encircling the axis of said rod below said heating member and spanning a substantial axial length of said rod below said heating member to limit the height of said zone,

((1) flow means for introducing cooling liquid into said heating member and said truncated cone for transport of heat therefrom,

(e) means for supporting a seed crystal for rotation on the axis of said rod and above the upper end of said rod and for movement toward and away from said rod, and

(f) means for moving said rod upward through said heating member for progressive transfer of semiconductor material from said rod to said crystal by way of said molten zone.

3. In a system for transfer, into the form of a dislocation-free crystal, of semiconductor material from a rod supported at the bottom thereof in vertical orientation in a controlled atmosphere onto a seed crystal supported for rotation above and at the axis of said rod and movable toward and away from said rod, the combination which comprises:

(a) means including a single-turn, annular, heating member encircling the axis of said rod and mounted for relative movement between said heating member and said rod, said heating member being energized by high frequency alternating current to establish a molten zone in said rod, and

(b) a cooled, hollow, upward-directed, truncated cone having a central cylindrical channel and encircling the axis of said rod below said heating member spanning a substantial axial length of said rod below said heating member to limit the height of said zone.

4. In a system for transfer, into the form of 'a dislocation-free crystal from a rod onto a seed crystal, the com bination which comprises:

(a) means for supporting said rod at the bottom thereof in vertical orientation in a controlled atmosphere,

(b) means for supporting and rotating said seed crystal above and at the axis of said rod,

(c) means for moving said seed crystal toward and away from said rod,

(d) means including a single-turn, annular, heating member encircling the axis of said rod and mounted for relative movement between said heating member and said rod, said heating member being of hollow, tubular configuration energized by high frequency alternating current to establish a molten zone in said rod,

(e) a hollow, truncated cone, the conical surface of which radiates outwardly from said heating member, encircling the axis of said rod below said heating member and spanning a substantial axial length of said rod below said heating member to limit the height of said zone, said truncated cone mounted for relative movement between said rod and said truncated cone, and

(f) flow means for introducing cooling liquid into said heating member and said truncated cone for transport of heat therefrom.

References Cited by the Examiner FOREIGN PATENTS 7/ 1963 Great Britain.

NORMAN YUDKOFF, Primary Examiner.

G. HINES,

Assistant Examiner. 

1. A SYSTEM FOR CRUCIBLE-FREE FLOAT ZONE FORMING OF A ROD OF SEMICONDUCTOR MATERIAL WHICH COMPRISES: (A) MEANS INCLUDING A SINGLE-TURN R.F. INDUCTION COIL POSITIONED IN A HORIZONTAL PLANE, (B) A HOLLOW QUARTZ TUBE EXTENDING THROUGH SAID COIL, (C) A CONDUCTIVE RING ENCIRCLING SAID TUBE IMMEDIATELY BELOW SAID COIL AND ADAPTED FOR FLOW OF LIQUID COOLANT THERETHROUGH AS A BODY WHICH IS TRUNCATED CONE, THE CONICAL SURFACE OF WHICH RADIATES OUTWARDLY FROM SAID R.F. INDUCTION COIL, (D) FIRST SUPPORTING MEANS FOR SAID ROD AT THE BOTTOM OF SAID TUBE, (E) SECOND SUPPORTING MEANS FOR A SINGLE CRYSTAL SEED OF SEMICONDUCTOR MATERIAL IN THE UPPER END OF SAID TUBE, (F) MEANS FOR ROTATING SAID SECOND SUPPORTING MEANS, (G) MEANS FOR MOVING SAID SECOND SUPPORTING MEANS RELATIVE TO SAID FIRST SUPPORTING MEANS ALONG THE AXIS OF SAID TUBE, (H) MEANS FOR MOVING SAID FIRST SUPPORTING MEANS RELATIVE TO SAID COIL, AND (I) MEANS FOR FLOWING A PREDETERMINED GAS THROUGH SAID TUBE. 